The words drifted across the frozen battlefield: 'Stille Nacht. Heilige Nacht. Alles Schlaft, einsam wacht'. To the ears of the British troops peering over their trench, the lyrics may have been unfamiliar but the haunting tune was unmistakable. After the last note a lone German infantryman appeared holding a small tree glowing with light. 'Merry Christmas. We not shoot, you not shoot.'

It was just after dawn on a bitingly cold Christmas Day in 1914, 90 years ago on Saturday, and one of the most extraordinary incidents of the Great War was about to unfold.

Weary men climbed hesitantly at first out of trenches and stumbled into no man's land. They shook hands, sang carols, lit each other's cigarettes, swapped tunic buttons and addresses and, most famously, played football, kicking around empty bully-beef cans and using their caps or steel helmets as goalposts. The unauthorised Christmas truce spread across much of the 500-mile Western Front where more than a million men were encamped.

According to records held by the World War One Veterans' Association, there is only one man in the world still alive who spent 25 December 1914 serving in a conflict that left 31 million people dead, wounded or missing.

Alfred Anderson was 18 at the time. Speaking to The Observer, Anderson has revealed remarkable new details of the day etched on history, including pictures of Christmas gifts sent to the troops.

His unit, the 5th Battalion The Black Watch, was one of the first involved in trench warfare. He had left his home in Newtyle, Angus, in October, taking the train from Dundee to Southampton, then a ferry to Le Havre.

He was happy, healthy and surrounded by most of his former school friends, who had all joined the Territorial Army together in 1912. In October 1914 they thought that they were at the start of an exciting adventure. But by the first Christmas of the war they had already experienced its horror and the death of young friends was commonplace.

On 24 and 25 December, Anderson's unit was billeted in a dilapidated farmhouse, away from the front line, so he did not participate in any football matches. 'We didn't have the energy, anyway,' he said. But he can still recall vividly what happened on Christmas Day 1914.

'I remember the silence, the eerie sound of silence,' he said. 'Only the guards were on duty. We all went outside the farm buildings and just stood listening. And, of course, thinking of people back home. All I'd heard for two months in the trenches was the hissing, cracking and whining of bullets in flight, machinegun fire and distant German voices.

'But there was a dead silence that morning, right across the land as far as you could see. We shouted "Merry Christmas", even though nobody felt merry. The silence ended early in the afternoon and the killing started again. It was a short peace in a terrible war.'

In some parts of the front, the ceasefire lasted several weeks. There are also numerous trench yarns, some possibly apocryphal, about the impromptu fraternising. One, detailed in Michael Jurgs's book The Small Peace in the Big War, involved a young private who was led to a tent behind German lines by an aristocratic officer and plied with Veuve Clicquot. In another tale, a barber supposedly set up shop in no man's land, offering a trim to troops from either side.

Now aged 108 and living alone in Alyth, Perthshire, Anderson still treasures the gift package sent to every soldier a few days before the first Christmas of the war from the Princess Royal. The brass box, which is embossed with a profile of Princess Mary, was filled with cigarettes.

It also contained a cream card, with 1914 on the front, which says: 'With best wishes for a happy Christmas and a victorious New Year, from the Princess Mary and friends at home.'

'I'd no use for the cigarettes so I gave them to my friends,' he said. 'A lot of the lads thought the box was worth nothing, but I said someone's bound to have put a lot of thought into it. Some of the boys had Christmas presents from home anyway, but mine didn't arrive on time.'

To his delight, he discovered that his most treasured possession - a New Testament given to him by his mother before he left for France and inscribed with the message: 'September 5, 1914. Alfred Anderson. A Present from Mother' - fitted the box perfectly.

He kept both in his breast pocket until 1916 when a shell exploded over a listening post in no man's land killing several of his friends and seriously injuring him.

'This is all I brought home from the war,' he said, showing the box and Bible, but forgetting about his beret with its famous red hackle, which is the first thing you see when you step into his home.

There are still many aspects of the war that Anderson finds difficult to talk about. 'I saw so much horror,' he said, shaking his head and gazing into the middle distance. 'I lost so many friends.'

He recalled one incident that gave him a 'sore heart'. When he was first home on leave, he visited the family of a dead friend to express his condolences. He knew them well but soon realised that he was getting a frosty reception. 'I asked if they were going to ask me in and they said no. When I asked why, they just said, "Because you're here and he's not". That was awful. He's one of the lads I miss most.'

Two years ago Prince Charles paid him a private visit after learning that he had served briefly as batman to the Queen Mother's brother, Captain Fergus Bowes-Lyon, who, along with hundreds of Mr Anderson's regimental colleagues, was killed at the Battle of Loos in 1915.

The seemingly invincible Anderson, who was awarded France's highest honour - the Légion d'Honneur - in 1998 for his services during the First World War, was recently in the rare position of witnessing one of his six children's golden wedding anniversaries. His children, he said, five of whom are still alive, are what keeps him going.

Alfred Anderson has spent 90 years trying to forget the war. But it has been impossible. So on Saturday he will look back. 'I'll give Christmas Day 1914 a brief thought, as I do every year. And I'll think about all my friends who never made it home. But it's too sad to think too much about it. Far too sad,' he said, his head bowed and his eyes filled with tears.